1. Field of the Invention
The present invention relates to current source correction circuits for correcting current mismatch in metal-oxide-semiconductor transistors due to process variation.
2. Art Background
In today's information age, there are great demands for fast high-resolution analog-to-digital (A/D) and digital-to-anolog (D/A) converters. These converters are used in digital audio systems, telecommunication, and precision data acquisition systems, just to name a few. Many of these converters are designed using current sources. However, due to process variation, there is mismatch between transistor current sources, and hence resulting in low conversion accuracy. Therefore, there is a need to correct this current mismatch in the transistor current sources.
Prior art disclosed by Groeneveld, Schouwenaars, Termeer and Bastiaansen, entitled "A Self-Calibration Technique for Monolithic High-Resolution D/A Converters," in IEEE Journal of Solid-State Circuits, Vol. 24, No. 6, December 1989, pages 1517-1522; and Manoli, entitled "A Self-Calibration Method for Fast High-Resolution A/D and D/A Converters," in IEEE Journal of Solid-State Circuits, Vol. 24, No. 3, June 1989, pages 603-608 described two apparatus for calibrating current source.
Groeneveld et. al. described an apparatus using a switching network to switch between two nodes: a reference current node and an output current terminal node. When the switch is at the reference current node, a reference current flows into a n-channel transistor and a charge is stored by the intrinsic gate-source capacitance. When the switch is at the output current terminal node, that same amount of charge is available at this node which is used for calibration. There are imperfections, as stated by the authors, due to changes in the gate voltage of the transistor during switching. Furthermore, there is a time limit the switch must be switched back to the reference current node to recharge so as to keep its output current within a specified range. This limits the frequency at which the circuit can be operated. In fact, the apparatus can only operate between a narrow range between 20 Hz and 20 kHz.
Manoli described an apparatus making use of a modified dual-slope method which altogether eliminates the need for calibration voltages. Although the approach presented is different from that of Groeneveld, nevertheless, the apparatus consists of a substantially large number of circuit modules and it occupies large chip area. Therefore, it is not feasible for mass production due to high cost. Furthermore, the apparatus is restricted to operate at frequency lower than 70 kHz